A surface acoustic wave (SAW for short) is a wave which propagates with energy concentration around the surface of an elastic body, and a filter element utilizing this wave is called “surface acoustic wave filter”.
In a case of a duplexer using a surface acoustic wave filter, for example, a transmitted signal passing through a transmission filter is basically directed to an antenna, but part of the signal can become leakage power that finds its way into a reception filter, which results in filter damage. Such a filter damage problem tends to be more serious especially in a multiple-mode type surface acoustic wave filter.
A surface acoustic wave filter disclosed in Patent Literature 1 comprises a plurality of IDT groups, each of which is composed of a plurality of IDTs connected in series to each other, with a floating electrode interposed between an input/output electrode and a ground electrode. In the IDTs that are arranged next to each other, out of adjacent electrode fingers of, respectively, one IDT and the other, one electrode finger is connected to the floating electrode and the other electrode finger is connected to one of the input/output electrode and the ground electrode. In this construction, the power handling capability of the surface acoustic wave filter acting as a multiple-mode type surface acoustic wave filter can be enhanced, thereby suppressing filter damage.
However, in the structure of the surface acoustic wave filter disclosed in Patent Literature 1, due to capacitive coupling in the adjacent part between the electrode finger connected to the floating electrode and the electrode finger connected to the input/output electrode, leakage of electric current is developed between the adjacent IDTs. As a result, the surface acoustic wave filter disclosed in Patent Literature 1 poses the problem of inadequate attenuation in a region outside the passband.
An object of the invention is to provide a multiple-mode type surface acoustic wave filter and a duplexer that suffer less deterioration in out-of-passband attenuation and are excellent in power handling capability.    Patent Literature 1: Japanese Unexamined Patent Publication JP-A 2006-311180